The present invention relates to a rotation detecting apparatus for detecting the rotation of the engine crankshaft or the like of an automobile, for example.
In the past, an apparatus for detecting the rotation of the engine crankshaft or the like of an automobile, for example, has been proposed in which a magnetic body having a coil wound thereon is arranged opposite to a rotating body and an alternating electromotive force is induced by changes in the magnetic flux caused by the projections and depressions on the rotating body to detect the rotation of the rotating body.
However, this type of apparatus is disadvantageous in that since the alternating electromotive force induced in the coil is influenced by the flux changes causing the electromotive force or the rotational speed of the rotating body, when the rotational speed of the rotating body is low, practically no alternating electromotive force will be induced and the detection of the rotation of the rotating body will become impossible.
To overcome the foregoing deficiency in the prior art, apparatus are known in the art comprising, as disclosed in U.S. Pat. No. 3,147,574 or U.S. Pat. No. 3,371,272, a differential transformer consisting of a primary coil disposed to be energized by an energizing source and two secondary coils, whereby the magnetic coupling of the transformer is changed in response to the revolution of a rotating body and the rotation is detected depending on whether the output voltage value of the differential transformer is greater than a predetermined value. A disadvantage of this type of apparatus is that the output voltage value of the differential transformer varies under the effect of the ambient temperature, thus making it impossible to detect the rotation of the rotating body accurately.